Moisture is known to react with so-called “acid gases”, such as hydrogen sulfide, carbonylsulfide, carbondisulfide and mercaptans (mercaptans are also referred to as thiols) to form a complex compound. (The term “acid gas” is used herein to denote either gas phase, liquid phase, or mixture of gas and liquid phases, unless the phase is specifically mentioned.)
One problem presents itself: if one is interested in producing acid gas standard compositions, in other words acid gases having a known concentration of one of these gases in a matrix or carrier fluid, then one must consider how to reduce or remove the moisture. Gas standards may have to have, and preferably do have, a long shelf life, since the standard acid gas may not be required immediately after production. A source of acid gas and/or matrix gas may contain a considerable amount of moisture. Therefore, the reduction or removal of moisture from the acid gas is important if the stability of the acid gas in the standard gas is to be maintained.
U.S. Pat. Nos. 5,255,445 and 5,480,677 describe processes for drying and passivating a metal surface to enhance the stability of gas mixtures containing one or more gaseous hydrides in low concentrations in contact therewith. The process comprises purging gas in contact with the metal surface with inert gas to remove the purged gas, exposing the metal surface to an amount of a gaseous passivating or drying agent comprising an effective amount of a gaseous hydride of silicon, germanium, tin or lead and for a time sufficient to passivate the metal surface, and purging the gaseous passivating agent using inert gas. Optionally, an oxidizing agent is applied after the third step to stabilize the adsorbed stabilizing agent. The patent also mentions prior known processes, such as saturation passivation, where the container is subjected to several cycles of evacuating and filling with a much higher concentration of the same gaseous hydride, prior to being filled with the low concentration hydride mixture of interest. The two patents do not mention or describe processes to passivate containers adapted to store sulfur-containing gases, nor do they mention passivation techniques in which a first passivating agent is applied to the surface, followed by contacting with a higher concentration of the gas to be stored.
Co-pending application Ser. No. 10/157,467, filed on May 29, 2002 describes the use of certain acid gas resistant molecular sieves to reduce or remove moisture from fluid compositions comprising a sulfur-containing compound. There is no disclosure or suggestion, however, for the passivation of containers adapted to contain the moisture-reduced compositions. Such containers may have moisture adhered to the internal surfaces, which can and does react with acid gases, reducing their stability and shelf-life.
A second, related problem involves the containers that the reactive gas standards are stored in. If metal or metal lined, reactive gases will react with and/or become adsorbed onto the metal, and will ultimately change the concentration of the reactive gas.
Grossman et al. (U.S. Pat. No. 4,082,834) describes alloys, such as alloys of nickel, titanium, and zirconium, that react with water and reactive gases (such as hydrogen, hydrogen-containing compounds such as hydrocarbons, carbon monoxide, carbon dioxide, oxygen, and nitrogen) at temperatures ranging from about 200° C. to about 650° C. While the patent does not discuss acid gases, it is apparent that hydrogen sulfide, carbonyl sulfide, and mercaptans are hydrogen-containing compounds, so that there would not be any expected benefits using these alloys to remove moisture from these acid gases. While carbondisulfide does not contain hydrogen, and therefore there could be some moisture reduction from a composition comprising carbondisulfide and moisture using these alloys, the high temperature is prohibitive for commercial use.
Tamhankar et al. (U.S. Pat. No. 4,713,224) describes a one-step process for removing minute quantities of impurities from inert gases, where the impurities are selected from the group consisting of carbon monoxide, carbon dioxide, oxygen, hydrogen, water and mixture thereof. The process comprises contacting the gas with a particulate material comprised of nickel in an amount of at least about 5% by weight as elemental nickel and having a large surface area, from about 100 to about 200 m2/g. There is no disclosure of removal of moisture from reactive gases; there is therefore no discussion or suggestion of moisture removal from reactive gases, moisture removal from matrix gases and mixing same to form a standard gas composition.
Tom et al (U.S. Pat. Nos. 4,853,148 and 4,925,646) discloses processes and compositions for drying of gaseous hydrogen halides of the formula HX, where X is selected from the group consisting of bromine, chlorine, fluorine, and iodine. The patent describes the use of, for example, an organometallic compound such as an alkylmagnesium compound, on a support. The halide is substituted for the alkyl functional group. Suitable supports are, alumina, silica, and aluminosilicates (natural or synthetic). However, there is no description or suggestion of reducing or removing moisture from sulfur-containing reactive gases, or of removal of moisture from matrix gases and mixing the reduced moisture gases to form a standard gas. Alvarez, Jr. et al. (U.S. Pat. No. 5,910,292) describes a process and apparatus for removal of water from corrosive halogen gases, using a high silica zeolite, preferably high silica mordenite. The patent describes removing moisture down to less than or equal to 100 ppb water concentration in halogen gases, particularly chlorine- or bromine-containing gases, but once again, there is lacking any teaching of suggestion of standard gas compositions. U.S. Pat. No. 6,183,539 discloses utilizing high sodium, low silica faujasite particles for the adsorption of carbon dioxide and water vapor from gas streams. The disclosed types of gas streams in which this type of high sodium, low silica faujasite crystals can be utilized includes air, nitrogen, hydrogen, natural gas, individual hydrocarbons and monomers, such as ethylene, propylene, 1,3 butadiene, isoprene and other such gas systems. There is no mention of sulfur-containing acid gas purification using the faujasites, or production of standard gas compositions.
U.S. Pat. No. 4,358,627 discloses use of “acid resistant” molecular sieves, such as that known under the trade designation “AW300”, for reducing the chloride concentration in chlorinated liquid hydrocarbons that contain an ethylenically unsaturated chlorinated hydrocarbon, water and hydrogen chloride. The method includes providing certain nitrogen-containing compounds in the system and contacting the system with the molecular sieve. There is no disclosure or suggestion, however, of removal or reduction of moisture from gas phase compositions, or production of standard gas compositions.
With respect to sulfur compounds, one impetus for measurement of low levels of these compounds has come from the hydrocarbon process industry where the requirement for lower sulfur measurements is steadily increasing. Controlling the concentration of sulfur is quite important for two major reasons. First, low sulfur content in simple olefin feed stock for high purity linear polymers such as polyethylene and polypropylene is quite important. Sulfur compounds adsorb onto the surface of the catalysts and prevent the desired reactants from reaching the catalytic surface thereby deactivating the catalyst. Given the quantity of these products produced annually, the economic impact can be quite significant. Therefore, common sulfur impurities in the feed stock, such as H2S (hydrogen sulfide) and COS (carbonyl sulfide) must be carefully monitored.
Second, sulfur in the form of sulfur dioxide (SO2), has been classified as a criteria pollutant by the EPA. This translates into strict regulation of emissions from coal burning plants, motor vehicle emissions, and paper and wood pulp processing plants to name a few. Sulfur in coal and gasoline becomes sulfur dioxide when the fuel is combusted. This SO2 is a precursor to acid rain. Reducing sulfur in gas not only lowers sulfur dioxide emissions but also reduces catalytic converter poisoning. This increases a vehicle's pollution control efficiency thus reducing other pollutants.
In order to monitor, control and regulate these compounds and other reactive gases, calibration gas mixtures containing these compounds must be used. For these mixtures to be utilized successfully, they must be reliable with respect to stability (shelf life) and concentration. Significant challenges arise as the concentrations of reactive gas mixtures required to calibrate equipment are decreased. Interactions on the cylinder and tubing surfaces, such as chemical reactions and adsorption, are often insignificant with reactive gas concentrations in the range of 10 ppm and above. These interactions become quite important with reactive gases at concentrations at or below 1 ppm. Impurities, such as moisture, which are present in the source gas can also play a major role in the stability of these mixtures.
Given the problem of moisture reacting with acid gases and reactive gases in general, it would be advantageous if passivation methods could be provided which increase the shelf-life during the storage of these compounds.